Add some more test coverage to verify the IDE plugins are consistent in how they behave in composite builds. This just covers a couple of simple scenarios. Change the `eclipse` task of the root project so that it generates the Eclipse projects for all builds, and so produces usable project files when there are included builds.
Fixes to ensure that `gradle :idea` generates all of the modules that are referenced in the IDEA project and so produces a functional IDEA project. Previously, this was working by assuming that a user would always run `gradle idea` and would never write or use any automation that happened to invoke `:idea`. This change fixes `gradle openIdea` to generate a functional IDEA project for multi-project builds. Added a bunch of test coverage to ensure that `gradle :idea` does not generate too much stuff as well, when modules are excluded from the project.